1. Field of the Art
The present invention generally relates to an outboard motor with a bracket assembly and, more particularly, relates to an outboard motor that has a bracket assembly to mount a drive unit of the outboard motor on an associated watercraft.
2. Description of Related Art
Typically, outboard motors incorporate a bracket assembly to mount a drive unit thereof on a transom of an associated watercraft. The bracket assembly typically includes a swivel bracket carrying the drive unit for pivotal movement about a steering axis that extends generally vertically, and a clamping bracket supporting the swivel bracket and the drive unit for pivotal movement about a tilt axis that extends generally horizontally. The drive unit usually has a propeller in a lower portion thereof to generate thrust that moves the watercraft. Typically, an engine disposed in an upper portion of the drive unit provides power to rotate the propeller through a drive mechanism disposed within the drive unit.
In general, the swivel bracket has a tubular section. The drive unit has a steering shaft extending through the tubular section of the swivel bracket. The steering shaft defines the steering axis. The tubular section has upper and lower bushings to pivotally support the steering shaft. The steering shaft thus can pivot within the tubular section so that the drive unit can move to the port side and to the starboard side about the steering axis. For example, Japanese Patent Publication Nos. JP11-245892A and JP11-310194A disclose such relationships between the tubular section of the swivel bracket and the steering shaft of the drive unit.
The lower portion of the drive unit is submerged under water while the propeller propels the associated watercraft. Under the circumstances, a floating object such as, for example, a piece of driftwood can strike the lower portion of the drive unit, or the drive unit can strike a rock under the water while the watercraft travels in shallow water. Relatively a large impact load is exerted on the bracket assembly in those situations. Even when such an impact load is not exerted, the bracket assembly experiences the thrust loading from the propeller as the propeller propels the associated watercraft.
The impact load or the thrust can generate a relatively large bending moment affecting a portion of the steering shaft between the upper and lower bushings. In order to prevent an outer surface of the steering shaft from contacting with an inner surface of the tubular section by the elastic deformation of the steering shaft caused by the bending moment, an outer diameter of the steering shaft can be slightly smaller than an inner diameter of the tubular section so that those surfaces are spaced apart from each other. Alternatively, both of the tubular section and the steering shaft can be made thicker to have relatively high rigidity or strength against the bending moment. Conventionally, the tubular section and the steering shaft are produced by a low pressure cast method using a shell core.
The space made between the tubular section and the steering shaft, however, can make the tubular section larger unless the steering shaft has a smaller outer diameter that is strong enough to endure the bending moment. Also, if the tubular section and the steering shaft are thicker, the outboard motor is inevitably cumbersome and the weight of the outboard motor inevitably increases.